(a) Field of the Invention
The present invention relates to a semiconductor device and a surface emitting semiconductor laser device, and more in particular to the semiconductor device and the surface emitting semiconductor laser device having a higher long-term reliability.
(b) Description of the Related Art
A surface emitting semiconductor laser device emitting light in a direction perpendicular to a substrate attracts public attention in the data communication field because of the possible arrangement of a plurality of the laser devices in a two-dimensional array on a single substrate, different from a conventional Fabry-Perot semiconductor laser device.
The surface emitting semiconductor laser device includes a pair of DBRs (Distributed Bragg Reflectors) (for example, Al(Ga)As/Ga(Al)As in the GaAs-based reflector) and an active layer acting as an emitting region sandwiched between the reflectors overlying a GaAs or InP substrate.
In order to increase the current injection efficiency and to reduce the threshold current, a surface emitting semiconductor laser device has been proposed having the current confinement structure made of an Al oxide layer.
As shown in FIG. 1, a conventional 850 nm-range surface emitting semiconductor laser device 10 includes a layer structure, overlying an n-GaAs substrate 12, having a bottom DBR mirror 14 having 35 pairs of n-Al0.9GaAs/n-Al0.2GaAs each having a thickness of xcex/4n (xe2x80x9cxcexxe2x80x9d is a lasing wavelength and xe2x80x9cnxe2x80x9d is a refractivity), a bottom cladding layer 16, a quantum well active layer 18, a top cladding layer 20 and a top DBR mirror 22 having 25 pairs of p-Al0.9GaAs/p-Al0.2GaAs each having a thickness of xcex/4n.
In the top DBR mirror 22, one of the layers close to the active layer 18 is formed as an AlAs layer 24 in place of the Al0.9GaAs layer, and Al of the AlAs layer 24 in the area other than a current injection area is selectively oxidized to form a current confinement area made of an Al oxide area 25 which surrounds the current injection area.
The top DBR mirror 22 in the layer structure is configured to be a circular mesa post 23 having a diameter of 30 xcexcm from the top to the layer near to the active layer 18 made of the photolithographic and etching process.
The current confinement area made of the Al oxide area 25 is formed around the mesa post 23 by selectively oxidizing the Al in the AlAs layer 24 inwardly from the outer periphery of the mesa post 23 by means of the oxidation treatment of the layer structure at about 400xc2x0 C. in a water vapor ambient.
When, for example, the Al oxide area 25 includes an annular ring having a width of 10 xcexcm, the surface area of the central AlAs area 24 or the surface area for the current injection (aperture) is about 802 xcexcm2 having a circular shape (diameter is 10 xcexcm).
The mesa post 23 is surrounded by, for example, a polyimide section 26, and a ring-shaped electrode acting as a p-side electrode 28 is mounted in contact with the periphery of the top surface of the mesa post 23 by the width from 5 xcexcm to 10 xcexcm. After the thickness of the n-GaAs substrate 12 is adjusted to about 200 xcexcm by polishing the bottom surface thereof, an n-side electrode 30 is formed thereon.
An electrode pad 32 for connection with an external terminal is mounted on the polyimide section 26 and in contact with the ring-shaped electrode 28.
In the surface emitting semiconductor laser device having the narrowed oxide area different from the current confinement structure for ion implantation, the formation of the mesa post 23 having the exposed periphery is required for the oxidation of the AlAs area in the p-type DBR mirror.
The height of the mesa post 23 or the etching depth of the top DBR mirror depends on the thickness of the top DBR mirror 22 because the periphery of the AlAs area 24 should be exposed for the oxidation.
For increasing the refractivity of the DBR mirror 22, 20 pairs or more of the compound semiconductor layers are necessary. Accordingly, the thickness of the top DBR mirror 22 amounts to about 4 xcexcm to 5 xcexcm.
As described earlier, the mesa post 23 is surrounded by the resin such as polyimide to flatten the step of the mesa post 23. As shown in FIGS. 2A to 2C, the mesa post 23 is entirely surrounded by the polyimide section 26.
The electrode pad 32 on the polyimide section 26 reduces the parasitic capacitance generated under the electrode pad more significantly than the electrode pad on the compound semiconductor layer, thereby providing the higher speed operation.
The surface emitting semiconductor laser device with the narrowed oxide area is frequently used for higher speed operation (modulation) because of the lower threshold current.
When the surface emitting semiconductor laser device having the mesa post surrounded by the polyimide is thermally treated at 300 to 400xc2x0 C. in the device manufacturing step (wafer process) or the mounting step (package), the stress is generated between the compound semiconductor layer and the polyimide section due to the difference between the thermal expansion coefficients thereof and is applied to the mesa post, thereby exerting ill effects on the active region. This is not preferable with respect to the initial characteristics and the reliability of the surface emitting semiconductor laser device.
Also in the actual operation circumstance in which the device operates in the temperature range between 0 and 85xc2x0 C., the stress generated between the polyimide section and the DBR mirror may exert ill effects on the long-term reliability of the surface emitting semiconductor laser device. Accordingly, the reduction of the stress generated between the polyimide section and the mesa post is requested.
A similar problem may arise in the surface emitting semiconductor laser device having the mesa post surrounded by the polyimide in addition to the above device having the narrowed oxide layer. Further, a similar problem may arise in a general semiconductor device including a light emitting device, a light receiving device and a transistor other than the surface emitting semiconductor laser device.
It is an object of the present invention to provide a semiconductor device and a surface emitting semiconductor laser device having a longer time of life which are obtained by reducing stress applied to a mesa post.
In a first aspect of the present invention, a semiconductor device is provided which includes a substrate, a mesa post overlying the substrate and having a substantially cylindrical shape, a resin member surrounding the mesa post and a stress moderating member received in the mesa post for moderating stress between the mesa post and the resin member.
In a second aspect of the present invention, a surface emitting semiconductor laser device is provided which includes a substrate and a layer structure formed thereon, the layer structure including a mesa post surrounded by a resin member having a stress moderating member for moderating stress between the mesa post and the resin member.
In accordance with the present invention, the stress applied to the mesa post of the semiconductor device or the surface emitting semiconductor laser device is reduced because the entire volume of the resin member is divided by the stress moderating member and each of the divided resin members reduces the stress.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.